There is a present need to decrease carbon dioxide (CO2) emissions from industrial facilities. Over the years, a number of electrochemical processes have been suggested for the conversion of CO2 into useful products. Processes for CO2 conversion and the catalysts for them are discussed in U.S. Pat. Nos. 3,959,094, 4,240,882, 4,349,464, 4,523,981, 4,545,872, 4,595,465, 4,608,132, 4,608,133, 4,609,440, 4,609,441, 4,609,451, 4,620,906, 4,668,349, 4,673,473, 4,711,708, 4,756,807, 4,818,353, 5,064,733, 5,284,563, 5,382,332, 5,457,079, 5,709,789, 5,928,806, 5,952,540, 6,024,855, 6,660,680, 6,664,207, 6,987,134 (the '134 patent), 7,157,404, 7,378,561, 7,479,570, U.S. Patent Application Publication No. 2008/0223727 (the '727 application) and papers reviewed by Hori (Modern Aspects of Electrochemistry, 42, pages 89-189, 2008) (“the Hori Review”), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, pages 1-19, 2006) (“the Gattrell review”), and DuBois (Encyclopedia of Electrochemistry, 7a, pages 202-225, 2006) (“the DuBois review”).
Generally an electrochemical cell contains an anode 50, a cathode 51 and an electrolyte 53 as indicated in FIG. 1. Catalysts are placed on the anode, and/or cathode, and/or in the electrolyte to promote desired chemical reactions. During operation, reactants or a solution containing reactants is fed into the cell. Then a voltage is applied between the anode and the cathode, to promote an electrochemical reaction.
When an electrochemical cell is used as a CO2 conversion system, a reactant comprising CO2, carbonate or bicarbonate is fed into the cell. A voltage is applied to the cell, and the CO2 reacts to form new chemical compounds. Examples of cathode reactions in the Hori Review includeCO2+2e−+2H+→CO+H2O2CO2+2e−→CO+CO32−CO2+H2O+2e−→CO+2OH−CO2+2H2O+4e−→HCO−+3OH−CO2+2H2O+2e−→H2CO+2OH−CO2+H2O+2e−→(HCO2)−+OH−CO2+2H2O+2e−→H2CO2+2OH—CO2+5H2O+6e−→CH3OH+6OH−CO2+6H2O+8e−→CH4+8OH−2CO2+8H2O+12e−→C2H4+12OH−2CO2+9H2O+12e−→CH3CH2OH+12OH−2CO2+6H2O+8e−→CH3COOH+8OH−2CO2+5H2O+8e−→CH3COO−+7OH−2CO2+10H2O+14e−→C2H6+14OH−CO2+2H++2e−→CO+H2O, acetic acid, oxalic acid, oxylateCO2+4H++4e−→CH4+O2 where e− is an electron. The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible cathode reactions.
Examples of reactions on the anode mentioned in the Hori Review include:2O2−→O2+4e−2CO32−→O2+2CO2+4e−4OH−→O2+2H2O+4e−2H2O→O2+4H++4e−The examples given above are merely illustrative and are not meant to be an exhaustive list of all possible anode reactions.
In the previous literature, catalysts comprising one or more of V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Sn, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, and Nd have all shown activity for CO2 conversion. Reviews include Hori (Modern Aspects of Electrochemistry, 42, pages 89-189, 2008) (“the Hori Review”), Gattrell, et al. (Journal of Electroanalytical Chemistry, 594, pages 1-19, 2006) (“the Gattrell review”), DuBois (Encyclopedia of Electrochemistry, 7a, pages 202-225, 2006) (“the DuBois review”), and the papers Li, et al. (Journal of Applied Electrochemistry, 36, pages 1105-1115, 2006, Li, et al. (Journal of Applied Electrochemistry, 37, pages 1107-1117, 2007) and Oloman, et al. (ChemSusChem, 1, pages 385-391, 2008) (“the Li and Oloman papers”), and references therein.
The results in the Hori Review show that the conversion of CO2 is only mildly affected by solvent unless the solvent also acts as a reactant. Water can act like a reactant, so reactions in water are different than reactions in non-aqueous solutions. But the reactions are the same in most non-aqueous solvents, and importantly, the overpotentials are almost the same in water and in the non-aqueous solvents.
The catalysts have been in the form of either bulk materials, supported particles, collections of particles, small metal ions or organometallics.
Co-owned U.S. patent application Ser. Nos. 12/830,338 and 13/174,365, as well as International Application Nos. PCT/US2011/030098 and PCT/US2011/042809, disclose a catalyst mixture comprising an active metal and a Helper Catalyst capable of catalyzing CO2 conversions with low overpotential and high electron conversion efficiency. However, the catalysts disclosed in these patent applications showed a lower activity than was desired.
The examples above consider applications for CO2 conversion, but the present electrochemical device overcomes limitations of other systems. For example, some commercial CO2 sensors use an electrochemical reaction to detect the presence of CO2. At present, these sensors require over 1-5 watts of power, which is too high for portable sensing applications.